This invention ralates to a power steering device for parking for automotive vehicles and, more particularly to such a device including a left-turn switch and a right-turn switch associated with the steering column. In case of the steering wheel's being unoperated, the switches can adopt a first position and in case of a left or a right turn of the steerig wheel, can adopt a second position. Each switch is associated with an AND-gate, the other input of which --via a further speed-AND-gate --is connected to a speed-responsive threshold value release switch and a further release switch. This electronic system acts upon an associated left turning angle valve and an associated right turning angle valve, which valves --upon the actuation of the left-turn switch or of the right-turn switch correspondingly connect a hydraulic pressure source to a steering-assisting hydraulic energy-generating device acting upon the steering linkage.
Such a power steering device for parking is switched on only when the vehicle's speed has dropped below a certain speed of, e.g., 5 km/hr and a further switch has been closed by, for instance, switching into the first gear or into the rear gear (German Published Patent Application DE-OS No. 34 10 033 corresponding to U.S. Pat. No. 4,616,727). Thus, when parking, an intensive additional energy source is available for the actuation of the steering system. So the driver has but to generate a small force for the torsion of the steering wheel, especially when being at a standstill.
During normal driving at higher speeds and in higher gears, however, the hydraulic steering aid unit for parking is switched off as now the steering system can be operated more easily as the driver has a better feeling for the steering behavior of his vehicle without hydraulic boosting of the steering force.
A problem in the known power steering device is that it is impossible to use normal switches with moving switch contacts as the strains ensuing during the torsion of the steering column are too small. Thus, for instance, it must be operated with wire strain gauges which must be connected up with a complicated and expensive electronic system. A further disadvantage is that the respective left-turn and right-turn switches are operated during any movement of the steering wheel even during normal driving.